National energy goals currently being pursued call for increased utilization of coal for industrial purposes in preference to oil and natural gas. This policy is intended to decrease U.S. dependence on foreign oil and to preserve natural gas supplies for residential use. Recent developments in the search for alternative fuels have been in the successful production of coal-water slurries which may contain 65 to 80 percent dry coal by weight suspended in water. The surprising characteristics of these slurries are that they exhibit good fluidity and stability both in transport and storage and are generally obtained by the use of pulverized coal of a particular particle size distribution for efficient particle packing coupled with the use of certain chemical additives to provide good fluidity and stability. In addition, the slurries can be beneficiated during production by decreasing the ash and sulfur contents of the coal. The substitution of coal-water slurry firing for oil in stream boilers which were originally designed for coal firing but which had been converted to oil and for boilers originally designed for oil firing would provide one solution to our national energy goals.
During the early 1980's, The Babcock & Wilcox Company, assignee of the instant invention, conducted combustion tests at its Alliance, Ohio Research Center to determine the suitability of coal-water slurries as a substitute fuel. A major program involved testing a slurry containing 66 percent solids by weight produced from a beneficiated high volative eastern bituminous coal, using compressed air as the atomizing fluid. The primary objective was to demonstrate that the slurry could be fired in a test unit of 4.0.times.10.sup.6 Btu/hr. nominal capacity with only minor modifications to the existing fuel oil handing and firing equipment.
Although the program was generally encouraging, some of the less desirable results can be summarized as follows:
1. Stable ignition could be maintained only in a limited firing range of 3.9 to 4.4.times.10.sup.6 Btu/hr. PA1 2. Combustion air temperature of 600 deg. F. was required. PA1 3. Preheating the furnace to operating temperature was needed for successful ignition. PA1 4. Flame temperatures during slurry firing were 200 to 400 deg. F. lower than the equivalent parent coal flame. PA1 5. Flue gas flow rates through the test unit were higher than flow rates using conventional pulverized coal because of the high water content of the slurry. PA1 a. The resulting reduction in gas velocity through the convection passes will reduce erosion of heating surfaces. PA1 b. Reduced exit gas flow from the unit will result in higher boiler efficiency. PA1 c. Forced and induced draft fan requirements will be reduced. PA1 d. Flame temperature will be higher to offset the depression of temperature due to the high water content of the slurry.
Clearly, there is a need for improvement in the combustion of coal-water slurries of high solids content particularly with respect to stable ignition over a range of fuel properties and in turndown capability corresponding to variable load demands.